Description will be made with reference to a conventional electrophotographic image forming apparatus such as a laser beam printer.
An ordinary the electrophotographic image forming apparatus forms an electrostatic latent image by irradiating an irradiating an electrophotographic photosensitive member, uniformly charged by a charging means, with light corresponding to image information, and visualize the electrostatic latent image as an image by supplying developer (hereinafter, referred to as “toner”) as a recording material by the use of a developing means. Further, the toner image is transferred from the photosensitive member onto a recording paper as a recording medium, and the recording paper P holding the toner is sent to a fixing device so as not to disturb the toner image, which image is then subjected to fixation under heat and pressure by the fixing device to be recorded and outputted as a permanent image on the recording paper. To the developing means, a toner container as a developer containing portion containing the toner is connected. The toner is consumed by forming the image. In many cases, the toner container, the developing means, the photosensitive member, the charging means and so on, are integrally constituted as a process cartridge (hereinafter, referred to as a “cartridge”). When the toner is consumed, a user can form again an image by replacing the cartridge with a new one.
In the cartridge, a predetermined amount of toner determined by a container volume is contained. Accordingly, the number of printable sheets by the user generally correlates with the amount of toner. Users who save the toner by reducing toner consumption to permit a larger number of printable sheets are also increased. In addition, laser beam printers having such an image formation mode, such as a low (toner) consumption mode, capable of automatically decreasing the amount of toner consumption or such a draft mode wherein printing is effected by converting image data to be printed into low resolution image data, image data decreased in the number of gradation levels, or image data decreased in image density are also increased.
As a means for decreasing the toner consumption amount, it is possible to use a means for changing a developing contrast, a means for changing a laser light quantity, etc. By changing the developing contrast or the laser light source, a latent image formed on the photosensitive member is changed. As a result, a toner coverage can be reduced at the time of development.
However, in the case where the amount of toner consumption is decreased only by the developing contrast or the laser light quantity, a thin line image or a character image has a very narrow line width to provide a poor image quality in some cases even under such a condition that a change in image quality is less conspicuous with respect to a solid black image having a large area to some extent.
For this reason, as a means for reducing the toner consumption amount while ensuring the line width, such a control method that an image frame portion constituted by a binary image is printed at an original density but an amount of toner consumption is decreased at an inner portion of the image, is performed to permit a decrease in toner consumption amount while ensuring the line width (e.g., Japanese Laid-Open Patent Application No. Hei 9-085993). More specifically, as shown in FIG. 3, the control method effects such an image processing that an original image (image data) 301 to be printed is changed into a dither image 302 wherein a frame portion, as a concentrated pixel area like a solid black image, is printed at an original density but an inner portion is provided with distributed blank dots which are not printed or a halftone image 303 wherein an amount of emission of laser or a laser on-period is changed on a one dot unit basis.
Herein, such an image formation mode for suppressing a toner coverage by changing an amount of emission or an emission period (or emission time) of a laser on one image dot unit basis is referred to as a “low (toner) consumption mode”.
However, the above-described conventional image control means is accompanied with the following problems.
The low toner consumption mode image processing method which has been conventionally used, as described above, a frame portion of a concentrated pixel portion of a resultant image is printed at an original density and the image is converted into a dither image or a halftone image at an inner portion (central portion) to reduce an amount of toner consumption. In this case, the image processing method is uniformly adapted to all the images except for those at the frame portion. A proportion between a pattern of the dither image or a pattern of the halftone image is switched according to the use circumstances, whereby it becomes possible to provide a low consumption mode which maintains image qualities.
However, in the case of effecting a low consumption mode with the dither image, when an amount of toner consumption is intended to be further decreased compared with that in the conventional low consumption mode, there arises such a problem that a blank dot portion is very conspicuous to make an image, to be originally a solid black image, a mesh image.
Further, in the case of effecting a low consumption mode with the halftone image obtained by changing an emission time or an emission light quantity of a scanner laser, there arises such a problem that the low consumption mode is more liable to be affected by a durability change of a photosensitive layer of the photosensitive member. More specifically, with respect to an ordinary laser light in the case where the halftone treatment is not performed, there is substantially no influence by a sensitivity change due to wearing or abrasion of the photosensitive layer caused by long-term use of the photosensitive member. However, with respect to a laser light changed in emission time or emission light quantity, a sensitivity of the photosensitive member becomes lower as the photosensitive layer becomes thinner with a progress of a durability change of the photosensitive layer, i.e., wearing or abrasion of the photosensitive member. As a result, a large density lowering and a deterioration in line width are caused to occur.
Further, it is possible to mount a density sensor for detecting a sensitivity change of the photosensitive member or an surface potential sensor for the photosensitive member to change the emission time or the emission light quantity on the basis of a detection result of the sensor, thus creating the halftone image. However, the mounting of the sensors is accompanied with a problem in terms of cost for incorporating detection circuits for the above-described sensors and a problem regarding ensuring of mounting space for mounting the sensors.
In addition, in the above-described pattern difference in area of image such as the solid black image or line width as in the conventional image control means, an amount of toner consumption required to maintain an image quality is different due to a difference in image area in response cases where the low consumption mode with the dither image is performed, so that it is necessary to sacrifice a decrease degree of the toner consumption amount if the toner consumption amount is uniformly decreased irrespective of image area.